1. Field of the Invention
The present invention relates to a novel ion exchanger of spherical beads based on cross-linked glucomannan and, more specifically, to an ion exchanger superior in compressive strength permitting to use under high hydrodynamic shear stress, for general ion-exchange purposes and, in particular, for ion-exchange chromatography.
2. Description of the Prior Art
As typical ion exchangers for ion-exchange chromatography, there are found in the market today DEAE-Sephadex A-50 (DEAE is an abridgement of diethylaminoethyl and is used below throughout the Specification), DEAE-Sepharose CL-6B, CM-Sephadex C-50 (CM is an abridgement of carboxymethyl) and CM-Sepharose CL-6B, all trademarks, of the firm Pharmacia Fine Chemicals. They are each produced by the use of a natural high molecular weight substance as the starting material and are used for separating high molecular weight substances, such as, proteins, polysaccharides, nucleic acids, membrane components and so on.
These prior art ion exchangers have a disadvantage that each bead of the ion exchanger becomes, due to lower compressive strength, deformed upon ion exchange operation at high flow velocity whereby a considerable reduction in the separation efficiency is caused. A further increase in the flow velocity may cause a steep rise of the pressure loss in the exchanger column and may result even in a blockade of the column.
In the case of using an ion exchanger for the separation of, such as, proteins, polysaccharides, nucleic acids, membrane components and other high molecular weight substances, it is necessary in general to provide for the subsequent adsoption step, namely ion exchange step, by removing all the substances combined by ionic bond.
In general, it is conventional to remove contaminants, such as lipids and proteins, by washing out with an aqueous NaOH solution of a concentration of about 0.1M, followed by a complete removal of the residual alkali by washing with distilled water, with a buffer solution or with a salt solution. This procedure is rather more laborious than the ionic adsorption procedure, requires longer time and, thus, constitutes a principal cause of the extended entire operation cycle composed of the process steps of ionic adsorption, desorption (elution), water wash and regeneration. It is therefore preferable in a practical operation to reduce the process periods in the washing and regeneration steps, rather than to reduce the process period in the adsorption step of the objective substance. In order to meet such a practical expectation, beads of ion exchanger having higher compressive strength and permitting a high liquid flow rate are to be provided.
On the other hand, it has been proposed in the Japanese Patent Application Lay-Open No. 236839/1987 to employ glucomannan beads, which have a cross-linked structure and exhibit a degree of swelling in the range from 1.5 to 15 in terms of number of ml of wet gel bed per g of dry gel, an exclusion limit molecular weight in the range from 200 to 100,000,000 and an average bead size in the range from 1 to 500 .mu.m, for the packing material in the techniques of gel filtration and gel chromatography. Glucomannan beads do not, however, have ion exchange capacity and, therefore, cannot be used for ion-exchange chromatography.